What are light waves waves in?

[Brightonian]

I'm almost embarrassed to ask this, but how can you have waves in a vacuum?

EDIT: of course the same Q applies to all kinds of visible or non-visible radiation.

[slaven41]

You don't have to be embarrassed. The greatest scientific minds in the world had the same question. Anyway, here's a stab at it. (Or two stabs at it, actually.)

Answer #1 - The "classical physics" answer: Light is an electromagnetic wave, which consists of oscillating electric and magnetic fields. These two fields feed off each other, in that a changing electric field produces a magnetic field and a changing magnetic field produces and electric field. And these fields can exist in a vacuum, so the waves can exist in a vacuum. (This isn't really a very good answer, as it leads to the question of how these fields can exist in a vacuum, or equivalently, how is "action at a distance" (in which two charged particles can influence each other with no intervening material) possible.)

Answer #2 - The "quantum physics" answer: Light is not really a wave, but is an entity which possesses "wave/particle duality," which means that it sometimes exhibits particle-like behavior and sometimes exhibits wave-like behavior. But it's not exactly a wave and it's not exactly particles. So what is it? Light is a quantum vector field. And if that doesn't give you much of a mental picture, well, it doesn't give me one either. That's really just a description of the mathematics that physicists use to calculate results. The point is, you just can't use any kind of mental picture for what light really is, because there's nothing else that works like quantum stuff works.

So anyway, there's two crappy, unenlightening answers for you. But it's all I've got.

--Dave

[Brightonian]

Thanks Dave, that's very helpful. The one thing that puzzles me is how it's taken me so long to even realize there's an issue here ...

[admin]

I like slaven41's answer a lot, but for what it's worth here are my thoughts on the subject.

In trying to understand the universe, we use analogy more often than reason. We call stuff "matter" if it is like a rock, and we call stuff "light" if it is like sunlight. The ancient Greeks thought about this a lot. Democritus thought matter was made up of indivisible "atoms", and was mostly right, except that atoms are themselves made up of smaller particles. Lucretius thought that "light" was a skin around every object, which the object kept shedding like a snake sheds its skin. He was almost entirely wrong on that subject, though he was amazingly right about the motion of falling objects, correcting Aristotle. It wasn't until modern times that people understood that light was electromagnetic in nature, with the electric and magnetic fields playing leapfrog. Because light could be polarized, the fields move up and down like...well like the waves in the ocean. Thus "light waves", which doesn't really say anything except light changes and the change has a direction. (Unlike sound waves, which compress and expand.) But light, like matter, has an indivisible kernel, a photon, so it is kind of like a particle, too.

Thus when physicists talk about the duel nature of light, they are saying it is kind of like a rock and kind of like the sea, but not really like either one. In fact, it is itself, and there is nothing else like it.

Then Einstein discovered that light can be changed to matter and matter to light, according to e = mc^2. So there is really just one "stuff" in the universe. If it travels really, really fast (at the speed of light) we call it light. If not so fast, we call it matter.

Now, I have a question. Electrons are matter, not light. But they have a wave-length. What's waving?

[slaven41]

Nice elaboration.

As for the question of what's waving in an electron waves...

When it's referred to at all, it's generally called an electron field, or something like that. As you pointed out, the distinction between matter and radiation that physicists used to make is largely artificial. That is, we can make the following analogy:

photon -> electromagnetic wave -> electromagnetic field -> Maxwell's equations
electron -> electron wave -> electron field -> Schrodinger equation

So, photons are particles whose behavior is governed by electromagnetic waves which consist of an oscillating electromagnetic field. This field is described mathematically by Maxwell's equations. Similarly, electrons are particles whose behavior is governed by an electron wave which consists of an oscillating electron field. This field is described mathematically by Schrodinger's equation.

As for why we don't have a special name for the electron field, as we do for the photon field, well that's largely a matter of history I think. By, for example, running electric charge up and down an antenna you can create massive amounts of photons, and you can detect the effect of this. This was done before anyone knew anything about quantum mechanics, and it got the name electromagnetic field.

There's no analogous process for electrons. That is, you can't build an antenna that creates enough electrons flying off in all directions to produce the overall effect of an easily measurable, continuous electron wave. (This is due, I think, partly to the fact that electrons interact with stuff differently from photons, and partly due to the fact that the electron has mass.) Therefore the electron field never got a name distinct from the particle.

--Dave

[admin]

Okay.

But...there are 4 (and only 4) "forces": gravity, electromagnetism, weak nuclear, and strong nuclear. In photons, the electromagnetic force waves. In electrons, what waves? Not the charge? Nor the mass nor the shell nor the energy nor the momentum. What's left? I thought if you described an electron by its quantum state, you had said everything there was to say about it. But is seems to me there has to be something else going on.

I had a similar problem with the statement that if you knew the atomic weight and atomic number and charge of an atom, you knew everything there was to know about the atom. In that case, I couldn't understand why some atoms fission and other atoms with the same numbers don't. Then, I saw a film of a uranium atom, and it looked more like jello than like a rock. It was wobbling all over the place, so of course some of the wobbles were going to make it split. But what's wobbling? Not the nucleus, that's too small to see. Not the electrons, they're trapped in shells. But something sure is wobbling, and least in the film I saw.

[slaven41]

Hmm. First off, I guess the distinction between matter and radiation isn't completely artificial. You can think of the Standard Model of particle physics as starting off with matter particles. These are the quarks, electrons, neutrinos, and a couple of other things. With only these things, however, none of the particles interact with each other. Interactions only come about through a second set of particles which include the photon, the W's and Z, gluons, and gravitons. And because interactions only take place because of these particles, they're said to generate the four forces.

But my point is that asking what is waving in the case of an electron is just like asking what is waving in the case of a photon. The oscillating electromagnetic field associated with a photon is no more "substantial" or "material" than the oscillating electron field associated with an electron. We just have a name for the electromagnetic field because we can observe its effects on a macroscopic scale.